The present invention relates to an analog/digital or a digital/analog converter. The device has a conversion device for converting an analog or digital input signal into a digital or analog output signal with respect to a specific reference voltage. The converter has an internal reference voltage selection device, to which a plurality of reference voltages are applied and which, depending on a selection signal, selects one of the reference voltages and applies the same to the conversion device.
In electronic metrology, i.e., electronic measurement technology, circuits in which a plurality of selectable reference voltages must be used (for example in the case of multi-channel ratiometric measurements in which the ratios of a plurality of voltages relative to one another are to be determined) have hitherto been constructed in a relatively complicated manner.
Conventional A/D or D/A converters, however, are provided with only one reference voltage input. In order to be able to carry out such ratiometric measurements (e.g. the comparison of two sensor voltages) with A/D converters having only one reference voltage input, at least two separate measurements and also subsequent formation of the ratio between the digital conversion results of the A/D converter are necessary. By contrast, if a plurality of freely selectable reference voltage inputs were present, the same ratiometric measurement could be handled in just a single measurement step, since one of the two sensor voltages could be used as reference voltage and the other sensor voltage could be used as analog voltage to be converted.
Conventional A/D and D/A converters can be operated with a plurality of reference voltages only when an external changeover of the reference voltage respectively supplied is provided, as a result of which, however, the accuracy is generally reduced.
Although a few cases of A/D converters which can internally alter the reference voltage respectively used by means of a resistor divider have already been disclosed, these A/D converters have the disadvantage that the reference voltage is loaded with a permanent direct current, this being the case in particular even when no A/D conversion is currently in progress.
Furthermore, the ratiometric measurements described above are not possible, even with A/D converters of this type. Moreover, these A/D converters are not calibratable, i.e. simultaneous correction of linearity and/or offset errors with the aid of a calibration operation is not possible. However, the accuracy of an A/D or D/A conversion is critically determined by linearity and offset errors which are caused by a mismatch of different circuit sections. In sensor technology, in particular, the signal voltages to be processed are very small, so that precisely in this area of application, high demands are placed on the accuracy of the A/D and D/A converters in order that corruption of the measurement results can be avoided or at least suppressed. In order to be able to comply with the rising accuracy requirements made of an A/D or D/A conversion, therefore, calibratable A/D or D/A converters and also powerful calibration methods are necessary, so that the errors caused by a mismatch can be compensated.
A/D and D/A converters with self-calibration are already widely known. Commonly assigned U.S. Pat. No. 5,825,316 (German patent DE 195 12 495 C1), for example, describes an A/D converter in which the conversion of an analog input signal into a digital output signal is performed according to the principle of charge redistribution and successive approximation. The principle of charge redistribution with successive approximation is also described in detail for example in U.S. Pat. No. 4,399,426 and also in xe2x80x9cAll-MOS Charge Redistribution Analog-to-Digital Conversion Techniques Part Ixe2x80x9d, James L. McCreary and Paul R. Gray, IEEE Journal of Solid State Circuits, December 1975 pages 371-79. The A/D converter accordingly comprises a main network, serving for the A/D conversion with a plurality of reference elements, in particular capacitors, whose capacitances are selected in a weighted manner. Furthermore, a correction network with likewise weighted capacitors is provided, which generates correction voltages for the correction of offset and/or linearity errors which are fed into the main network.
However, even in these known A/D and D/A converters with self-calibration, the use of a plurality of different reference voltages is not known, i.e. the reference voltage is constant throughout operation. The reference voltage cannot be changed between individual conversions. This also means, however, that a calibration which possibly precedes a conversion, the conversion itself and a calibration which possibly succeeds the conversion are carried out with the same reference voltage. A calibrating A/D or D/A converter with a reference voltage which can be selected for each conversion is not known.
It is accordingly an object of the invention to provide an A/D converter or a D/A converter, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and thus to provide an A/D or D/A converter whose reference voltage can be freely selected, even during operation. In particular, the present invention is based on the object of proposing a calibratable A/D or D/A converter of this type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an analog/digital or digital/analog converter, comprising:
a conversion device for converting an analog input signal into a digital output signal, or converting a digital input signal into an analog output signal, with respect to a specific reference voltage;
the conversion device having an internal reference voltage selection device connected to receive a plurality of reference voltages and configured to select, in dependence on a selection signal, one of the reference voltages and apply the selected reference voltage to the conversion device; and
wherein the plurality of reference voltages are freely selectable reference voltages, and the specific reference voltage defining the conversion is freely selectable.
In other words, the A/D or D/A converter has an integrated internal selection device to which different reference voltages are fed and which selects one of these reference voltages for the A/D or D/A conversion depending on a control signal. This internal selection device may be configured in particular in the form of an analog multiplexer which can be driven via a data bus with the control signal. In this case, the changeover of the reference voltage that is respectively to be used is effected by transmission gates, so that the reference voltages are not additionally loaded by the changeover operation.
In a preferred exemplary embodiment, the A/D or D/A converter according to the invention comprises a correction or calibration circuit which is suitable, on the one hand, for operation with different freely selectable reference voltages and, on the other hand, for both offset and linearity calibration. The calibration circuit comprises, in particular, a plurality of weighted reference elements, for example capacitors, resistors or transistors, those reference elements at which the positive reference voltage is present during the zero point respectively selected being provided twice, namely once for the offset calibration and once for the linearity calibration. In the context of the present invention, a specific procedure is proposed with regard to the application of the different voltages to the reference elements of the calibration circuit, resulting in the possibility of reliable compensation of both offset and linearity errors using one and the same calibration circuit with the simultaneous use of a plurality of freely selectable reference voltages.
In accordance with an added feature of the invention, the conversion device comprises a main network with a plurality of weighted reference elements and an output, and a comparator connected to the output of the main network, and a correction network with weighted further reference elements is coupled to the main network for correcting offset errors and linearity errors, and wherein the reference elements of the main network are assigned correction values for driving the correction network.
In accordance with an additional feature of the invention, the main network is configured to convert the input signal according to the principle of charge redistribution, and the weighted reference elements in the main network and the correction network are capacitors.
In accordance with another feature of the invention, a main network controller sets a voltage to be applied in each case to the reference elements of the main network, and a correction network controller sets a voltage to be applied in each case to the reference elements of the correction network.
In accordance with a further feature of the invention, at least one reference voltage of the plurality of reference voltages applied to the reference voltage selection device is a temporally constant reference voltage, and the correction network controller applies the temporally constant reference voltage to the correction network as a base reference voltage for correcting offset errors and linearity errors.
In accordance with again an added feature of the invention, the correction network is allocated a specific zero point at which the correction network controller applies in each case either the base reference voltage or a negative reference voltage to the individual weighted reference elements of the correction network.
In accordance with again an additional feature of the invention, all the reference elements of the correction network to which the base reference voltage is applied at the zero point are divided into a corresponding offset reference element for correcting offset errors and into a corresponding linearity reference element for correcting linearity errors.
In accordance with again another feature of the invention, the zero point of the correction network is defined such that the base reference voltage is applied only to a most significant reference element of the correction network by the correction network controller at the zero point, while the negative reference voltage is applied to all other reference elements of the correction network at the zero point.
In accordance with again a further feature of the invention, the correction network controller, for a correction of offset errors, fixedly applies the base reference voltage to the linearity reference element, while in a sample phase of the comparator, the base reference voltage or the negative reference voltage is applied to the offset reference element and the other reference element of the correction network depending on a previously determined offset correction value, and, in a decision phase of the comparator the base reference voltage is applied to the at least one offset reference element and the negative reference voltage is applied to the other reference elements of the correction network, the comparator storing the voltage present at a node between the main network and the correction network in the sample phase and converting the voltage into a new offset correction value in the decision phase.
In accordance with yet again a further feature of the invention, the correction network controller, for a correction of linearity errors, in a sample phase of the comparator applies the base reference voltage or the negative reference voltage to the offset reference element and the other reference elements of the correction network depending on a previously determined offset correction value and applies the base reference voltage to the linearity reference element, while in a decision phase of the comparator, the base reference voltage is applied to the offset reference element and either the base reference voltage or the negative reference voltage is applied to the linearity reference element and also the other reference elements of the correction network depending on a previously determined linearity correction value, and wherein the comparator stores a voltage present at a node between the main network and the correction network in the sample phase and converts the voltage into a new linearity correction value in the decision phase.
Finally, in accordance with a concomitant feature of the invention, the main network is configured to convert the input signal according to the principle of charge redistribution, and the weighted reference elements in the main network and the correction network are capacitors, the correction network controller is configured to, during a conversion of the converter, in a sample phase thereof, apply either the base reference voltage or the negative reference voltage to the offset reference element and the other reference elements of the correction network depending on a previously determined offset correction value and to apply a reference voltage, which is instantaneously selected by the reference voltage selection device, to the linearity reference element, while in a subsequent charge redistribution phase of the converter, the base reference voltage is applied to the at least one offset reference element and either the reference voltage, which is instantaneously selected by the reference voltage selection device, or the negative reference voltage is applied to the linearity reference element and also the other reference elements of the correction network depending on a previously determined linearity correction value.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a analog/digital or digital/analog converter, which, in principle, can be applied to both A/D and D/A converters (for example in microcontrollers), it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.